There are four main factors affecting enzyme activity:-
1. Temperature: enzymes work best at an optimum temperature.
Below this, an increase in temperature provides more kinetic energy to the molecules involved. The numbers of collisions between enzyme and substrate will increase so the rate will too.
Above the optimum temperature, and the enzymes are denatured. Bonds holding the structure together will be broken and the active site loses its shape and will no longer work.
2. pH: as with temperature, enzymes have an optimum pH. If the pH changes much from the optimum, the chemical nature of the amino acids can change.
This may result in a change in the bonds and so the tertiary structure may break down. The active site will be disrupted and the enzyme will be denatured.
3. Enzyme concentration: at low enzyme concentration there is great competition for the active sites and the rate of reaction is low. As the enzyme concentration increases, there are more active sites and the reaction can proceed at a faster rate.
Eventually, increasing the enzyme concentration beyond a certain point has no effect because the substrate concentration becomes the limiting factor.
4. Substrate concentration: at a low substrate concentration there are many active sites that are not occupied. This means that the reaction rate is low.
When more substrate molecules are added, more enzyme-substrate complexes can be formed. As there are more active sites, and the rate of reaction increases.
Eventually, increasing the substrate concentration yet further will have no effect. The active sites will be saturated so no more enzyme-substrate complexes can be formed.
Inhibitors
Inhibitors slow down the rate of a reaction. Sometimes this is a necessary way of making sure that the reaction does not proceed too fast, at other times, it is undesirable.
Reversible inhibitors:
Competitive reversible inhibitors: these molecules have a similar structure to the actual substrate and so will bind temporarily with the active site. The rate of reaction will be closer to the maximum when there is more ‘real’ substrate, (e.g. arabinose competes with glucose for the active sites on glucose oxidase enzyme).
Non-competitive reversible inhibitors: these molecules are not necessarily anything like the substrate in shape. They bind with the enzyme, but not at the active site. This binding does change the shape of the enzyme though, so the reaction rate decreases.
Irreversible inhibitors:
These molecules bind permanently with the enzyme molecule and so effectively reduce the enzyme concentration, thus limiting the rate of reaction, for example, cyanide irreversibly inhibits the enzyme cytochrome oxidase found in the electron transport chain used in respiration. If this cannot be used, death will occur.
Cofactors
Most enzymes require additional help from cofactors, of which there are 2 main types:
1. Coenzymes - these are organic compounds, often containing a vitamin molecule as part of their structure.
Coenzymes are not permanently bound to the enzyme but may be temporarily and loosely bound for the duration of the reaction and then move away once it is completed. For example NAD, which transfers hydrogen away from one molecule in a dehydrogenase reaction and takes it to another molecule (see the Respiration QuickLearn).
2. Metal ions - most speed up the formation of the enzyme-substrate complex by altering the charge in the active site e.g. amylase requires chloride ions, catalase requires iron.
Examples of the industrial uses of enzymes
Perhaps the best known use is that of protease in biological washing powders. This enzyme helps to break down protein stains such as blood at lower washing machine temperatures. This means they save energy and are gentler on clothes. Some people are allergic to biological washing powders but this may be overcome by encapsulating the enzymes in wax from which they are only released during the wash.
Another wide spread use of enzymes is that of pectinases in food modification. Pectin is a substance which, is found in cell walls and helps to hold the structure together. Pectinase is the name given to a group of enzymes which, break down pectins. They are therefore used to partially digest fruit and vegetables in baby food and to help extract fruit/vegetable juices.
